Sample preparation strategies for petroleum coke digestion and further cerium and lanthanum determination by DSN-ICP-OES

The determination of cerium and lanthanum in petroleum coke samples by inductively coupled plasma optical emission spectrometry coupled to a desolvating nebulizer system (DSN-ICP-OES) is proposed for the first time after studying the following sample preparation methods: microwave-assisted digestion (MW-AD) in medium and high pressure systems and microwave-induced combustion (MIC). MW-AD in a medium pressure system was not suitable for digestion of petroleum coke even when using a low sample mass (up to 100 mg). Both MIC and MW-AD in a high pressure system were considered suitable for petroleum coke digestion, but MIC was preferable in view of the possibility of using diluted acid (2.5 mol L⁻¹ HNO₃) to digest 400 mg of sample and obtaining lower residual carbon content (RCC) values (below 0.1% instead of approximately 25% for MW-AD). The use of MW-AD required a dilution step (with a factor of four) to minimize the interference in analyte determination by DSN-ICP-OES. Accuracy was evaluated by comparing the results with those obtained by neutron activation analysis (an agreement of more than 94% was obtained) and also analyzing spiked samples (recoveries in the range of 88 to 102% were achieved).